The present invention relates to an ultrasonic motor and a stator for an ultrasonic motor.
As shown in FIG. 4, a typical progressive wave type (or bolted Langevin type) ultrasonic motor includes a stator 51 and a rotor 52. The stator 51 includes first and second blocks 53, 54, which are made of metal, first and second piezoelectric elements 55, 56, first to third electrode plates 57 to 59, and a tightening member, which is a bolt 60. The first and second blocks 53, 54, the first and second piezoelectric elements 55, 56, and the first to third electrode plates 57 to 59 are piled in layer to form a substantially columnar shape. The first and second blocks 53, 54 are tightened by the bolt 60, which is inserted through the first and second blocks 53, 54 in the axial direction. This couples the first and second blocks 53, 54, the first and second piezoelectric elements 55, 56, and the first to third electrode plates 57 to 59.
A slit, which is not shown, is formed at the outer circumference of the lower portion of the stator 51, or the outer circumference of the second block 54. The slit generates torsional vibration based on the axial vibration.
The rotor 52 is substantially cylindrical and is rotatably pressed against the upper surface of the stator 51, or the upper surface of the block 53, by a pressing mechanism, which is not shown.
When high-frequency voltage is applied to the first to third electrode plates 57 to 59, the first and second piezoelectric elements 55, 56 generate axial vibration. Then, the torsional vibration is generated at the slit of the second block 54. The axial vibration of the stator 51 causes levitation force, and the torsional vibration causes driving force. The levitation force and the driving force cause the rotor 52 to rotate.
The first and second blocks 53, 54 are assembled by tightening the male screw of the bolt 60 to the female screws of the first and second blocks 53, 54. This determines the positions of the first and second blocks 53, 54 in the radial direction. Since the positions of the first and second blocks 53, 54 and the bolt 60 are determined only by the male and female screws, the first and second blocks 53, 54 could be misaligned. Therefore, manufacturing deviations are caused per stator 51, which causes variations in the rotational characteristics (such as frequency-rotational speed characteristic, voltage-torque characteristic, and the like) per product. Therefore, the rotor 52 cannot be used for purposes in which a high-accuracy rotational control is required (such as for rotating a drum in a copying machine). This limits the field of application of the ultrasonic motor.
Accordingly, it is an objective of the present invention to provide an ultrasonic motor that has a simple structure and reduces misalignment of metal blocks and a tightening member, and a stator for the ultrasonic motor.
To achieve the above objective, the present invention provides an ultrasonic motor, which includes a stator and a rotor. The stator includes a pair of metal blocks, a piezoelectric element, a tightening member, and a positioning member. The piezoelectric element is located between the metal blocks. When drive voltage having a predetermined frequency is applied to the piezoelectric element, the piezoelectric element vibrates the stator. The tightening member is inserted through the metal blocks and the piezoelectric element to tighten the metal blocks and the piezoelectric element in the axial direction. The positioning member determines the radial position of the metal blocks. The rotor is press fit to the stator and rotates in accordance with the vibration of the stator.
The present invention also provides a stator located in an ultrasonic motor, which includes a pair of metal blocks, a piezoelectric element, a tightening member, and a positioning member. The piezoelectric element is located between the metal blocks. When drive voltage having a predetermined frequency is applied to the piezoelectric element, the piezoelectric element vibrates the stator. The tightening member is inserted through the metal blocks and the piezoelectric element to axially tighten the metal blocks and the piezoelectric element. The positioning member determines the position of the metal blocks in the radial direction.